This application relates to and incorporates herein by reference Japanese Patent Application No. 11-97713 filed on Apr. 5, 1999.
The present invention relates to a GPS receiver which receives radio signals from GPS (global positioning system) satellites and detects its position from the received radio signals.
The GPS signals are widely used to detect positions of moving bodies such as automotive vehicles. Specifically, a GPS satellite in the air is searched for from the present time and the present position of a GPS receiver mounted on an automotive vehicle. The signal reception frequency is estimated from the orbit information of the GPS satellite in consideration of influence of Doppler effect. If three or more GPS satellites thus searched for are acquired, it becomes possible to execute a positioning processing, that is, to calculate the position of the automotive vehicle by using satellite orbit data included in GPS signals (satellite data) transmitted from the acquired GPS satellites.
It is necessary to synchronize the positioning processing with the satellite data. That is, time data included in the satellite data is extracted, and the distance between the satellite and the GPS receiver is calculated based on the extracted time and the present time of the GPS receiver. The position of the GPS satellite is calculated from the detailed orbit information (ephemeris) included in the satellite data. This positioning processing is executed with respect to three (preferably four) satellites, so that the present position of the automotive vehicle may be calculated from the position and data transmission time of each satellite.
The detailed orbit information is generally effective for about four hours. It is therefore not necessary to newly collect the detailed orbit information even when a power supply to the GPS receiver is shut down for a short period of time, as long as the detailed orbit information is stored in a backup memory provided in the GPS receiver. As a result, the GPS receiver is enabled to restart the positioning processing immediately from its power-on after the short power-off period, because it is only necessary for the GPS receiver to extract time information from the satellite data. This operation-restarting is generally referred to as a hot start. It is however still necessary to synchronize the positioning processing of the GPS receiver with the satellite data, when the time information is to be extracted in the hot start processing. That is, it is necessary to accurately determine the start point of the satellite data.
The satellite data is generally formatted to have five sub-frames in one frame. One sub-frame (6 seconds) includes ten words. The head word in each sub-frame is a TLM word, and the following word is a HOW (hand-over word). Thus, TLM and HOW appear every 6 seconds which is a time interval of each sub-frame during transmission of data.
It is necessary to check synchronization of data with respect to each sub-frame for the positioning processing of the GPS receiver. That is, it is necessary to check a preamble code provided in the TLM included at the head of the sub-frame, a sub-frame ID (identification code) and parity included in HOW, and the like. As a result, the synchronization determination processing of each frame takes at least 6 or more seconds which corresponds to one sub-frame. That is, the check is executed by retrieving the preamble code and the parity which appear every 6 seconds. In this instance, retrieving TLM and HOW takes 1.2 seconds, because one word corresponds to 0.6 second. Thus, it takes at least 7.2 seconds to retrieve the preamble codes and the parities of two frames for checking agreement of data included in the satellite data transmitted from each GPS satellite.
It is therefore an object of the present invention to provide a GPS receiver which is capable of synchronizing its positioning processing with satellite data thereby to shorten a time period required to start the positioning processing.
According to the present invention, a GPS receiver executes satellite frame synchronization determination processing to establish a synchronization of satellite data transmitted from a plurality of GPS satellites. Specifically, in the satellite frame synchronization determination processing, it checks an agreement among predetermined data included in the transmitted satellite data, when the plurality of GPS satellites the detailed orbit information of which are stored in a memory are acquired with in a predetermined time period.
Preferably, the GPS receiver executes a single-satellite frame synchronization determination processing, when it cannot establish the satellite frame synchronization or when it cannot acquire the plurality, of GPS satellites the detailed orbit information of which are stored in the memory cannot be acquired with in the predetermined time period. In the single-satellite frame synchronization determination processing, the GPS receiver checks for an agreement among predetermined data of a plurality of frames included in the satellite data transmitted from a predetermined one of the satellites and for establishing a frame synchronization of the satellite data.
Further preferably, the GPS receiver stores a general orbit data extracted from the satellite data and a present position, and searches for the GPS satellites based on the stored general orbit data, the present position and present time.